This invention relates to a series of volatile formation electrolytes for tantalum pellets, and more particularly to electrolytes volatile at 200.degree. C. which need not be rinsed out of the pellets subsequent to anodization.
Anodized tantalum pellets enjoy widespread use as anodes in solid-electrolyte capacitors. The dielectric oxide film of such capacitors is generally formed by anodic oxidation of tantalum pellets suspended from stainless steel or aluminum alloy bars into an electrolyte.
Typical prior art electrolytes consist of dilute aqueous solutions of mineral acids or the salts of mineral acids such as phosphoric, sulfuric, nitric, or hydrochloric acid. In the case of anodizations in sulfuric or phosphoric acid (or their salts), the electrolyte residues in the tantalum pellets after anodization have high boiling points. The removal by heating of these residues at 200.degree. C. or less is thus not possible. To remove the residues by heating to higher temperatures will result in damage to the oxide film, leading to high DC leakage currents in finished capacitors. Therefore, the method of cleaning tantalum pellets anodized in these prior art electrolytes generally involves extended rinsing in water, the duration of which varies according to pellet size (the larger pellets requiring longer rinses).
While anodization of tantalum pellets to high voltages may occur in these prior art acid electrolytes, aluminum bars are not compatible. The aluminum bar may be anodized in aqueous solutions of the salts of phosphoric acid (but not of sulfuric acid): however, the same problem of solute residue cleanup remains.
In the case of prior art anodization of tantalum pellets in aqueous nitric or hydrochloric acid, no extended rinsing procedure is necessary, because the electrolyte residues are volatile at 200.degree. C. However, the useful voltage range of tantalum pellets in these electrolytes is limited to 100 V. This limitation is caused by the effect of the electrolyte becoming highly concentrated in the pores of the pellet during anodization. Since nitric and hydrochloric acid electrolytes behave more poorly with respect to maximum formation voltage as their concentrations are increased, a self-limiting useful range is created. Additionally, these acids (and their salts) are not compatible with aluminum alloy bars, because, during attempted anodization, aluminum dissolution, not oxide growth, is the primary anodic process.
If these strong acids and their salts are thus eliminated from consideration as volatile anodizing electrolytes for tantalum and aluminum alloy bars, the choice becomes quite restricted. Weak acids and their salts--such as boric acid and borates--cannot be used because the conductivity of the acid is so low that oxide growth in the pellet interior is severely retarded.
Thus, the potential candidates for volatile anodizing electrolytes for tantalum pellets are limited to acids (and their salts) of the proper strength, such that the following conditions are fulfilled: first, that the concentrated free acid which is formed in the pores of the pellet during anodization is of sufficient conductivity that anodization of the pellet interior will occur; and second, that the concentrated free acid which is formed in the pellet pores can support the desired formation voltage. Also, the acid (or its salts) should be cleanly volatile at temperatures less than or equal to 200.degree. C., to eliminate extended post-anodization rinsing.
It has been recognized in the prior art that the acids in formation electrolytes for tantalum pellets optimally have ionization constants greater than 10.sup.-5, in order to maintain the correct environment for uniform tantalum pellet formation.